As a white LED, a two-color mixing LED which combines a semiconductor light-emitting element that emits blue light and a yellow fluorescent material to obtain white light by mixing colors of blue and yellow is widely used. However, the white light emitted by the two-color mixing white LED has a problem of low purity. Therefore, recently, a three-color mixing white LED which obtains white light by mixing colors of blue, green and red by combining a semiconductor light-emitting element that emits ultraviolet light (wavelength of 350 to 430 nm) and three types of fluorescent materials of blue, green and red and exciting the respective fluorescent materials with light from the semiconductor light-emitting element has been developed.
Conventionally, blue fluorescent materials having various compositions have been developed. For example, Patent Literature 1 describes an SMS blue light-emitting fluorescent material having a basic composition formula represented by Sr3-xMgSi2O8:Eux (x is 0.008 to 0.110), having the same crystal structure as merwinite and having a crystal lattice distortion of 0.055% or less. It is described that this SMS blue light-emitting fluorescent material shows high emission intensity and high stability to heat. Also, in the examples of this literature, a blue fluorescent material having a crystal lattice distortion of 0.031 to 0.066% is described.
Also, Patent Literature 2 describes a blue light-emitting fluorescent material having a basic composition formula of (Sr,Ca)3MgSi2O8 activated by Eu and having a merwinite crystal structure, wherein the molar ratio of Sr to Ca is in the range of 1:0.10 to 1:0.30. It is described that a part of Sr is replaced with Ca so as to achieve above-described molar ratio, whereby the emission intensity under a high temperature environment around 120° C. (100 to 150° C.) is high, and the temperature stability of emission intensity is improved. In addition, it is described that the blue fluorescent material of this literature is produced by drying a slurry of raw material powder with a rotary evaporator.
Furthermore, Patent Literature 3 describes a blue fluorescent material including a compound represented by Me3MgSi2O8:Eu (wherein Me is at least one of Ca, St and Ba). In the example of this literature, a blue fluorescent material (sample No. 3) that is (Sr,Ca)3MgSi2O8:Eu (Ca/Sr=0.1/1, x=0.1) and produced by a spraying method is described. It is described that the spraying method is a method of producing an aqueous solution containing raw materials for a fluorescent material by spraying it in a heated oven, and the mixed solution of raw materials is heated to 1500° C. while applying supersonic waves from a nozzle and sprayed.